Optimized central cutter and method

ABSTRACT

A central cutting structure for use with a drill bit includes a plurality of cutters, of which at least one overlaps the center of a supporting member from which the cutters extend. Each cutter has a side rake angle that provides an overlapping relationship with at least one other cutter to provide an aggressive cutting surface for penetrating hard formations.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to the co-pending U.S. patent application having the Ser. No. 12/218,832, filed Jul. 18, 2008, the entirety of which is incorporated herein by reference.

FIELD

The present disclosure relates, generally, to central cutting structures, usable in conjunction with a drill bit, to provide a portion of the drill bit that normally penetrates through a formation with reduced efficiency with a more optimal angle of attack, especially with respect to harder and/or more abrasive formations.

BACKGROUND

PDC (Polycrystalline Diamond Compact) drill bits were introduced in the oil and gas industry in the mid 1970's. During the past 30 years, numerous technological improvements to PDC cutters and drill bits have enabled them to take an important and growing share of the drilling bit market. In 2003, about 50% of the total footage drilled was performed using PDC bits, compared to 26% in 2000. Further in 2003, the total revenue of PDC bit sales was about $600 million.

Despite improvements in drill bit hydraulics and stability, and tougher and more abrasion resistant cutting elements, PDC drill bits experience a significantly reduced average rate of penetration and usable life when used in harder and more abrasive formations. Therefore historically, the use of PDC bits has been restricted to soft to medium and nonabrasive formations.

Of particular concern is the inability of the central portion of the bit face of a PDC drill bit to cut effectively. During use, the central cutters rotate a shorter distance per revolution than the outer cutters, causing less efficient boring of the drill bit as a whole. The comparative inefficiency of the central region of the bit is further accentuated when boring through a hard and/or abrasive formation.

While improvements have been made in the quality and variety of the cutters, such as new manufacturing techniques to prevent cutter wear and breakage, and improved impact and abrasion resistant diamond material and the interface geometry between the diamond layer and the tungsten carbide substrate, the high inefficiency of the central cutters of a drill bit has continued to provide difficulty.

PDC drill bits bore through a formation by shearing, like the cutting action of a lathe, as opposed to roller cone bits that drill by indenting and crushing rock. The PDC bit's cutting action plays a major role in the amount of energy needed to drill a rock formation, and can be modeled by studying the interaction between a single PDC cutter and the rock formation. Many models have been developed during the past 30 years to predict the forces on the PDC bit. The single cutter-rock models generally take into account the PDC cutter characteristics (cutter size, back rake angle, side rake, chamfer, etc.) and properties of a formation to calculate the forces necessary to cut an amount of rock. The 2D or 3D rock-bit interaction model takes into account the bit characteristics (profile, cutter layout, gauges) and the bit motion to calculate the Weight On Bit (WOB), Torque On Bit (TOB) and side force on the bit at given operating conditions in a given rock formation, either isotropic or heterogeneous formations. Laboratory single-cutler tests and full scale PDC bit tests have been carried out at atmospheric pressure or under bore-hole conditions and tend to validate these models, enabling many advances made in bit design and optimization.

The design of a PDC bit is largely a compromise between many factors, such as drillability, rate of penetration, hydraulics, steerability and durability. Typically, the design emphasizes the three parts of the PDC bit that interacts with the rock formation: the cutting structure (bit profile and cutter layout characteristics), the active gauge (gauge cutters or trimmers), and the passive gauge (gauge pads). There are three basic types of PDC bit profile: flat or shallow cone, tapered or double cone and parabolic, according to IADC fixed cutter drill bit classification there are nine bit profile codes. The type of profile plays an important role for the bit stability and durability and bit directional responsiveness. The choice of bit profile depends on the type of application, and it is difficult to give or apply general rules. Nevertheless, it is generally thought that the bit cone tends to make the bit more stable and that very flat profiles are generally used for sidetrack applications.

Thus, the characteristics of PDC cutters, i.e. back rake angle, cutter layout, cutter count, and cutter size, are the main parameters that control the drillability of the bit. The back rake angle is defined as the angle of the cutter face with respect to the rock. The back rake angle controls how aggressively the cutters engage the rock formation. Generally, as the back rake is decreased, the rate of penetration increases, however the cutter also becomes more vulnerable to impact breakage. A large back rake angle will typically result in a lower rate of penetration, but will also result in a longer PDC bit life. The side rake angle generally affects the cleaning of the cutters, by directing the cutting toward the periphery of the bit.

A need exists for a PDC drill bit or similar type of drill bit having a central cutting structure with improved efficiency, drilling with effectiveness at the center portion of the bit comparable to that at the extremities of the bit face.

A need also exists for a PDC drill bit or similar type of drill bit having an efficient attack angle with respect to the portion of the formation adjacent the center of the bit.

A further need exits for a removable and replaceable central cutting structures for use within the center of a the bit face of a PDC drill bit or similar type of drill bit.

Embodiments usable within the scope of the present disclosure meet these needs.

SUMMARY

Embodiments usable within the scope of the present disclosure include central cutting structures for use with a drill bit, such as a PDC drill bit having a bit body and bit face, and drill bits incorporating use thereof.

A central cutting structure can be fixedly secured to the center of a bit face, or removably secured thereto to facilitate replacement thereof should the central cutting structure and/or the drill bit become damaged and/or worn. The central cutting structure can include a support member having an attachment member extending from a side. In an embodiment, the attachment member can include a cylindrical body adapted for engagement within a complementary receptacle of the bit face.

Opposite the attachment member, a plurality of cutters can extend from the support member, each cutter having a cutting surface with an inner edge extending from the support member toward the inner edge of each other cutter, to define an apex of the central cutting structure. The cutting surface of each cutter can be include a side rake angle, such as −15 degrees to 15 degrees that provides an overlapping relationship with the cutting surface of at least one of the other cutters. The configuration of the cutting surfaces provides an aggressive cutting structure for penetrating hard or abrasive formations, despite the smaller amount of rotation per revolution inherent in the central region of the bit face.

While the configuration of the cutters can vary, in an embodiment, one or more of the cutters can overlap the center of the support member. Further, one or more of the cutters can have a base surface that extends radially between the center and edge of the support member. Additionally, one or more cutters can include an outer edge, such as curved, arcuate, and/or angled edge, that extends from the apex of the cutting structure toward the edge of the support member. Moreover, while any number of cutters can be disposed on the support member, in an embodiment, the central cutting structure can include three or more cutters.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate certain embodiments of the invention and together with the general description given above and the detailed description given below, describe numerous embodiments usable within the scope of the present disclosure.

FIG. 1 is a diagrammatic illustration of a prior art drill bit illustrating an angle of attack that is inefficient for removing formation using the central cutters, with respect to direction of rotation.

FIG. 2 is a cross-sectional side view of a cutter of a prior art drill bit, illustrating various rake angles that may be selected for engaging a formation.

FIG. 3 is a diagrammatic illustration of an embodiment of a central cutting structure usable within the scope of the present disclosure.

FIG. 4 is a diagrammatic illustration of an alternate embodiment of a central cutting structure usable within the scope of the present disclosure.

FIG. 5 is a perspective view of the embodiment of the central cutting structure illustrated in FIG. 3.

FIG. 6 is a side view of the embodiment of the central cutting structure shown in FIG. 5.

FIG. 7 is a top view of the embodiment of the central cutting structure shown in FIG. 5.

FIG. 7A is a top view of the embodiment of the central cutting structure shown in FIG. 7, with the cutting elements removed.

FIG. 8 is a perspective view of the embodiment of the central cutting structure shown in FIG. 5, in association with an adjacent cutter element.

FIG. 9 is a perspective view of the alternate embodiment of the central cutting structure illustrated in FIG. 4.

FIG. 10 is an elevated perspective view of the embodiment of the central cutting structure shown in FIG. 9.

FIG. 11 is a top view of the embodiment of the central cutting structure shown in FIG. 9.

FIG. 12 is a side view of an embodiment of the central cutting structure similar to that shown in FIG. 9, with the cutting elements removed.

FIG. 13 is a top view of the embodiment of the central cutting structure shown in FIG. 12.

FIG. 14 is a side view of an alternate embodiment of a central cutting structure, similar to that shown in FIG. 12.

FIG. 15 is a top view of the embodiment of the central cutting structure shown in FIG. 14.

FIG. 16 is a perspective view of the embodiment of the central cutting structure shown in FIG. 9, in association with an adjacent cutter element.

FIGS. 17-18 depict side views of embodiments of cutter elements usable with the embodiment of the central cutting structure of FIG. 5.

FIGS. 19-22 depict embodiments of a cutter element usable with the embodiment of the central cutting structure shown in FIG. 9, with FIG. 19 depicting a perspective view of the cutter and FIGS. 20-22 depicting various side views of the cutter.

FIG. 23 is a chart illustrating an embodiment of a method usable within the scope of the present disclosure.

The above general description and the following detailed description are merely illustrative of various embodiments of the invention, and additional modes, advantages, and features of this invention will be readily suggested to those skilled in the art without departing from the spirit and scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments described and depicted in the accompanying drawings.

As identified above, there exists a long-standing problem in bit design associated with the inefficiency of central cutters of any drill bit. Due to the working size of a typical PDC cutter, the possible positions of the central cutters are limited to a very inefficient angle for engaging the formation. This results in side rake angles that penetrate a formation more slowly than cutters proximate to the extremity of the drill bit, and in some cases, the center of the drill bit does not drill at all.

In soft to moderately hard rock, the inefficiency of the central cutters of the bit does not normally pose a significant issue. However, in hard, abrasive formations, such as, for example, sandstone within the Travis Peak formation, an advantage can be gained by improving the drilling efficiency of the center of the bit.

FIG. 1 is an illustration of a prior art drill bit illustrating an inefficient angle of attack for removing formation using the central cutters, with respect to direction of rotation. In a hard rock formation, the deficiencies of the central cutters are especially apparent, and can cause damage to the drill bit.

FIG. 2 is a cross-sectional side view of a prior art cutting element illustrating various rake angles in which the aggressiveness of a PDC-type cutter may be altered with respect to how it engages a formation. As shown in FIG. 2, the back rake angle of a gage cutter 40 may can include a zero rake angle 10, a positive rake angle 20 or a negative rake angle 30. Embodiments usable within the scope of the present disclosure can include side cutters 40A, 40B, 40C preferably positioned at an angle ranging from zero rake 10 to a negative rake 30. For some applications, a negative rake 30 is effective in a variety of formations 50. As shown in FIG. 2, the cutting surface 42 of the cutter 40A, 40B, 40C having a negative rake angle 30 and moving in the direction noted by arrow 44 is impacted by forces indicated by the arrow 60 at an angle of incidence 46 which is equal to ninety degrees plus the amount of cutter rake. In the depicted example, the angle of incidence 46 is about fifty-three degrees. The aggressiveness of the cutter 40 at least partially a function of the angle of incidence 46, which is generally at a maximum at a zero rake 10 angle, and at a minimum when negative rake 30 angle of minus ninety degrees is used.

It is common in the art to design bits with many different types of cutter layouts or distribution patterns. What is common to each of these patterns is that there are between one and four central cutters whose spatial disposition is severely inefficient. This severe lack of sufficiency is due to the fact that in the central part of the bit, a 0.5″ diameter cutter can only be optimized with respect to attack angle for a small portion of its diameter. Thus, there will be parts of the cutter with an efficient attack angle, and parts having an acceptable attack angle, and parts with an inherently poor attack angle. This phenomenon normally disappears an inch or two outside of the central portion of the bit.

As described previously, in soft or even moderately hard formations, this phenomenon rarely occurs, as the rock is either too soft or too brittle to cause this type of effect. Portions of the formation proximate to the center of the bit either break off as they becomes too tall to support themselves, or are broken or worn off simply by the drill bit body material rubbing against them. However, in hard, abrasive formations with high rock strength, inefficient cutting at the center of the bit can severely slow the rate of penetration.

FIG. 3 is an illustration of an embodiment of a central cutting structure having two cutters thereon, which shows that the angle of attack is very efficient for removing formation with respect to the central cutters, in the direction of rotation noted by the arrows. As can be seen in FIG. 3, this central cutter includes various attack angles much closer to the optimal angle for engaging the formation. The depicted cutting structure is shown having two adjacent, but opposed, diamond tables, which in use, leave a minimum of the formation uncut. The illustration shows four data points: (1) a −3.0° angle at a 0.5 inch radius from the center, (2) a 0.7° angle at a 0.375 inch radius from the center, (3) a 3.0° angle at a 0.250 inch radius from the center, and (4) a 10° angle at a 0.125 inch radius from the center. Any small, uncut portion of the formation will be dislodged by the PDC elements during rotation.

FIG. 4 is an illustration of an alternate embodiment of a central cutting structure having three cutters thereon, which shows that the angle of attack is very efficient for removing formation with respect to the central cutters, in the direction of rotation. FIG. 4 depicts a central cutting structure usable with a bit having three blades that merge toward the center of the bit face. The illustration shows four data points: (1) a −6.0° angle at a 0.5 inch radius from the center, (2) a 1° angle at a 0.375 inch radius from the center, (3) a 3.0° angle at a 0.250 inch radius from the center, and (4) a 11° angle at a 0.125 inch radius from the center. The illustrated attack angles are much closer to the optimal angle for engaging the formation, and more normalized with respect to the cutter rotation. Small areas of uncut formation that may remain in the center of the cutting structure are sufficiently small that lateral movements of the drill bit due to vibrations of a bottomhole assembly can remove any such uncut portion of the formation.

FIG. 5 depicts a perspective view of the central cutting structure 200 illustrated diagrammatically in FIG. 3. The depicted central drill bit structure 200 includes an end portion 210, a central member 220 and the two cutter supports 230. The cutter supports 230, in conjunction with a joining surface 222, support cutting elements 250. The cutting elements 250 have an exterior surface 256, which can include a diamond-cutting surface 280 thereon. Also, the cutting elements 250 are shown having a base surface 252 that engages the joining surface 222 associated with the central member 220 of the structure 200. The two side surfaces 232 are slightly overlapped with respect to the cutting elements 250. The support 230 is shown having a sloping surface 236 with an engaging surface 234 that supports and secures an engaging surface 254 of the cutting element 250.

FIG. 6 depicts a side view of the central cutting structure 200 of FIG. 5. The central cutting structure 200 is shown having the central member 220 with a joining surface 222, a cutter support 230, and cutting elements 250. The diamond-cutting surface 280 of one of the cutting elements 250 is also visible in FIG. 6.

FIG. 7 depicts a top view of the central cutting structure 200 of FIG. 5. The central cutting structure 200 is shown having the joining surface 222, cutter support 230, and cutting elements 250. Both of the depicted cutting elements 250 are shown having diamond-cutting surfaces 280. A gap 281 is shown between the cutting elements 250. The gap 281 provides an angled relationship between the cutting elements 250, such that an inner edge of each cutting element meets at the apex 280A of the cutting elements 250. The angled relationship can increase overlap of the cutting elements 250 from the apex 280A to the joining surface 222.

FIG. 7A depicts a top view of the central cutting structure 200 of FIG. 7 having the cutting elements removed. Of note, alternate sided, concave, arcuate angles 238 are shown along an edge of the joining surface 222. The depicted alternate sided, concave, arcuate angles 238 have an arc of approximately one hundred twenty degrees. Additionally, alternate sided, convex, arcuate angles 239 are shown adjacent to the convex angles 238. The alternate sided, convex, arcuate angles 239 are also depiocted having an arc of approximately one hundred twenty degrees.

FIG. 8 depicts a perspective view of the central cutting structure 200 of FIG. 5 in association with an adjacent cutter element A. The central cutting structure 200 is shown having the joining surface 222, cutter support 230, and cutting elements 250 having diamond-cutting surfaces 280 thereon. As described previously, the angled relationship of the cutting elements 250 provides for increased overlap from the apex 280A of the cutting elements 250 to the joining surface 222.

FIG. 9 is a perspective view of the embodiment of the central cutting structure 300 illustrated diagrammatically in FIG. 4. The depicted central cutting structure 300 is shown having an end portion 310, a central member 320, and three cutter supports 330. The cutter supports 330, in conjunction with a joining surface 322, support the cutting elements 350. The cutting elements 350 are shown having a diamond-cutting surface 380 disposed thereon. Also, the cutting elements 350 are shown having a base surface 352 that engages the joining surface 322 associated with the central member 320 of the structure 300. The side surfaces of each cutting element 350 are shown slightly overlapped with respect to each other cutting element 350. The support 330 is shown having a structure similar to that shown in FIGS. 5-7, which includes a sloping surface with an engaging surface that supports and secures the cutting elements 350. The depicted slope and configuration of the cutting elements 350 provides the diamond-cutting surface 380 of each cutting element with a side-rake angle of approximately zero degrees. Other embodiments can include a side-rake angle ranging from −15 degrees to 15 degrees.

FIG. 10 depicts an elevated perspective view of the central cutting structure 300 of FIG. 9. The cutter supports 330 in conjunction with the joining surface 322 support the cutting elements 350. The cutting elements 350, as described previously, have an exterior surface on which the diamond-cutting surface 380 is disposed. Also, the cutting elements 350 have a base surface 352 that engages the joining surface 322 associated with the central member 320 of the structure 300. The side surfaces of the cutting elements 350 are slightly overlapped with respect to each other cutting element. The support 330, as described previously, has a sloping surface with an engaging surface that supports and secures the cutting elements 350.

FIG. 11 depicts a top view of the central cutting structure 300 of FIG. 9. The central cutting structure 300 is show having the joining surface 322, cutter support 330, and cutting elements 350. Each of the three depicted cutting elements 350 are shown having diamond-cutting surfaces 380. A gap 381 is shown between the cutting elements 350, the gap 381 providing an angled relationship between the cutting elements 350 such that there is a match at the apex 380A of the cutting elements 350. The angled relationship enables for increased overlap between the cutting elements 350 from the apex 380A of to the joining surface 322.

FIG. 12 depicts a side view of an embodiment of a central cutting structure 400 similar to that shown in FIG. 9, having the cutting elements removed. The depicted central cutting structure 400 includes an end portion 410, a central member 420, and a cutter support 430.

FIG. 13 depicts a top view of the central cutting structure 400 of FIG. 12, with the cutting elements removed. The depicted central cutting structure 400 is shown having a joining surface 122 supporting a cutter support 430. The cutter support 430 can have at least two sides 436, 432. The depicted central cutting structure is also shown having three concave arcs 421 in the perimeter of the central member 420. The depicted concave arcs 421 are shown having an angle of approximately one hundred twenty degrees, however it can be appreciated by those skilled in the art that modifications to the depicted configuration are possible without departing from the scope of the present disclosure.

FIG. 14 depicts a side view of another embodiment of a central cutting structure 500, similar to that shown in FIGS. 9 and 12, having the cutting elements removed. The depicted central cutting structure 500 is shown having an end portion 510, a central member 520, and a cutter support 530.

FIG. 15 depicts a view of the central cutting structure 500 of FIG. 14. The central cutting structure 500 is shown having a generally round joining surface 522, and cutter supports 530 having sides 532, 534, and 536.

FIG. 16 depicts a perspective view of the central cutting structure 300 of FIG. 9, in association with an adjacent cutter B. The central cutting structure 300 is shown having a joining surface 322, a cutter support 330, and cutting elements 350. The three depicted cutting elements 350 are show having diamond-cutting surfaces 380. The angled relationship of the cutting elements 350 provides for increased overlap from the apex 380A of the cutting elements 350 to the joining surface 322.

FIG. 17 depicts a side view of an embodiment of a cutter 230 usable within a central cutting structure, such as that illustrated in FIG. 5. The cutter 230 is shown having sides 230A, 230G, 230F, 230E, 230J. FIG. 18 is an alternate, rotated side view of the cutter 230 of FIG. 17, showing sides 230A, 230B, 230C, 230D, 230E.

FIG. 19 depicts a perspective view of an alternate embodiment of a cutter 430 usable within a central cutting structure, such as that illustrated in FIG. 9. The cutter 430 is shown having sides 430A, 430B, 430C, and 430D. FIGS. 20-22 depict various side views of the cutter of FIG. 19

FIG. 23 depicts a chart illustrating an embodiment of a method usable within the scope of the present disclosure.

All of the embodiments as well as those appreciated by one skilled in the art after appreciating this disclosure allow for placing cutters immediately adjacent to and overlapping, within a central structure attachable to a drill bit. In an embodiment, the central cutting structure can be made from sintered tungsten carbide and/or a tungsten carbide matrix material, having polycrystalline diamond compact cutters affixed and/or bonded thereto.

In a further embodiment, embodiments of the central cutting structure can be cast as an integral part of a drill bit during the drill bit molding process, and then specialized cutter shapes can be brazed or otherwise attached thereto. Additionally, while the depicted embodiments include two and three cutting elements extending from a central cutting structure, it should be understood that embodiments can readily include any number of cutting elements, e.g. four or five cutting elements.

Additionally, the central cutting structures disclosed herein can allow a single brazing operation to be performed at the center of a drill bit to replace two, three, or more separate cutters with a single, higher efficiency cutting structure.

Additional advantages and modification will readily occur to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details, representative apparatuses, and the illustrative examples shown and described herein. Accordingly, departures may be made from the embodiments herein without departing from the spirit or scope of the disclosed general inventive concept. 

1. A central cutting structure for a drill bit, the central cutting structure comprising: a base member having a surface; and a plurality of cutters secured to the surface, wherein each of said cutters comprises a cutting surface having an inner edge, wherein the inner edge of each of said cutters extends away from the surface and proximate to the inner edge of each other of said cutters to define an apex, and wherein the cutting surface of each of said cutters has a side rake angle that provides an overlapping relationship with the cutting surface of at least one other of the cutters to provide an aggressive cutting structure for penetrating hard formations.
 2. The central cutting structure of claim 1, wherein the cutting surface of at least one of said cutters overlaps a center of the base member.
 3. The central cutting structure of claim 1, wherein the cutting surface of each of said cutters further comprises a base edge extending from a center of the surface toward an edge of the surface.
 4. The central cutting structure of claim 1, wherein the plurality of cutters comprises at least three cutters.
 5. The central cutting structure of claim 1, wherein the side rake angle ranges from −15 degrees to 15 degrees.
 6. The central cutting structure of claim 1, wherein the cutting surface of at least one of said cutters further comprises an outer surface extending from an edge of the surface to the apex.
 7. The central cutting structure of claim 1, further comprising an attachment member disposed on a side of the base member opposite the surface, wherein the attachment member is configured for engagement with a bit face of the drill bit.
 8. The central cutting structure of claim 7, wherein the attachment member is configured for removable engagement with the bit face of the drill bit.
 9. The central cutting structure of claim 7, wherein the attachment member is configured for fixed engagement with the bit face of the drill bit.
 10. The central cutting structure of claim 7, wherein the attachment member comprises a cylindrical body configured for engagement within a complementary receptacle within the bit face.
 11. A drill bit for forming boreholes in subterranean formations, the drill bit comprising: a drill bit body having a bit face with a center; a central cutting structure secured to the center of the bit face, wherein the central cutting structure comprises: a base member having a first side, a second side, a center, and an edge; an attachment member extending from the first side and engaged with the center of the bit face; a plurality of cutters extending from the second side, wherein each of said cutters comprises a cutting surface having an inner edge, wherein the inner edge of each of said cutters extends away from the second side proximate to the inner edge of each other of said cutters to define an apex, and wherein the cutting surface of each of said cutters has a side rake angle that provides an overlapping relationship with the cutting surface of at least one other of the cutters to provide an aggressive cutting structure for penetrating hard formations.
 12. The drill bit of claim 11, wherein the central cutting structure is removably engaged with the bit face.
 13. The drill bit of claim 11, wherein the central cutting structure is fixedly engaged with the bit face.
 14. The drill bit of claim 11, wherein the plurality of cutters comprise polycrystalline diamond compact cutters.
 15. The drill bit of claim 11, wherein the attachment member comprises a cylindrical body configured for engagement within a complementary receptacle within the bit face.
 16. The drill bit of claim 11, wherein the cutting surface of at least one of said cutters overlaps a center of the base member.
 17. The drill bit of claim 11, wherein the cutting surface of each of said cutters further comprises a base edge extending from the center of the base member the edge of the base member.
 18. The drill bit of claim 11, wherein the plurality of cutters comprises at least three cutters.
 19. The drill bit of claim 11, wherein the side rake angle ranges from −15 degrees to 15 degrees.
 20. The drill bit of claim 11, wherein the cutting surface of at least one of said cutters further comprises an outer surface extending from the edge of the base member to the apex.
 21. A method for penetrating a hard or abrasive formation with a drill bit, the method comprising the steps of: providing a central cutting structure to a center of a bit face of a drill bit, wherein the central cutting structure comprises a plurality of cutters extending therefrom, wherein each of said cutters comprises a cutting surface having an inner edge, and wherein the inner edge of each of said cutters extends away from the bit face and proximate to the inner edge of each other of said cutters to define an apex; and contacting the hard or abrasive formation with the central cutting structure, wherein the cutting surface of each of said cutters has a side rake angle that provides an overlapping relationship with the cutting surface of at least one other of the cutters to provide an aggressive cutting structure for penetrating the hard or abrasive formation. 